Process for the recovery of acid gases



Dec. 22, 1936. w R KNAPP 2,064,838

PROCESS FOR THE RECOVERY OF ACID GASES Filed Feb. 28, 1934 W QZEVENTORBY Patented Dec. 22, 1936 PATENT OFFICE PROCESS FOR THE RECOVERY OF ACIDGASES Walter R. Knapp, Pelham, N. Y., assignor, by mesne assignments, toHorvitz Patent Holding Corporation, New York, N. Y., a corporation ofNew York Application February 28, 1934, Serial No. 713,311

Claims.

This invention relates to a process for the recovery of acid gases fromgas mixtures and for the removal of acid gas impurities from industrialgases and its novelty consists in the steps of the process.

There are many acid gases having a commercial value that are, daily lostwith gaseous mixtures passing to the atmosphere that at present areconsidered only'as waste gases of commercial 10 industries.

There is no process except the process covered by United States PatentNo. 1,916,980 available to save, recover or clean these valuable acidgases, that is commercially practicable.

l5 My process furnishes a practical economical method of recovering acidgases from many chemical and industrial operations making such acidgases available at a low cost and also furnishes a process for thepurification of industrial gases from said gas at a low cost.

This process has a higher efficiency of removal and recovery of saidacid gases than any of the processes now in commercial use and requiresrelatively a smaller amount of external heat, the I amount of heatvarying somewhat with the nature of the acid gas removed or recovered.

. The process may be applied to the separation and recovery of suchgases ascarbon-dioxide, sulphur dioxide, hydrogen cyanide, hydrogensulphide onany other gas of acid reaction either organic or inorganicfrom waste gases. The process may also be applied to the removal of acidgases from industrial gases such as coal gas, water gas, or cracker gasas produced in oil refining operations where said acid gases arepresagrammatic figure showing the operation of the process as applied tothe recovery of carbon di- 50 oxide gas from gaseous mixtures such asthe waste gases of combustion from a steam boiler.

plant.

It will be understood that the various parts of theapparatus forcarrying out the process as 55 described are merely indicated in adiagrammatic manner in order to show the complete cycle of operationinvolved in this particular example of the process as described andclaimed.

The various parts of the apparatus are named in accordance with theparticular function of the 5 apparatus used in carrying out the processand the chemical elements and reactions resulting in chemical changesare named at the particular portion of the apparatus where the changesoccur, so that an examination of the diagrammatic 10 drawing willindicate clearly and definitely the particular apparatus used and theconnection between the same and the chemical elements and combinationsin each part of the apparatus.

Referring to the diagrammatic drawing, the 5 hot waste gas mixture istaken from the breeching or smoke stack of a commercial steam boilerplant (not shown). It will be understood that this source of supply isused only as an example and that this invention is not restricted tothat 20 source of supply but that other sources of supply may be used,such as gases of fermentation, gases from kilns, gases from smelting andother acid gases besides carbon dioxide can be so recovered. Such awaste gas mixture is taken by 25 means of apositive type exhauster orfan (not shown) through a cooler or condenser (not shown), through asulphur dioxide absorber (not shown) through a dirt, soot and fly ashremoval apparatus. (not shown) and enters a carbon di- 0 oxide absorberI through the inlet pipe 2 located near the base of the carbon dioxideabsorber. It then passes through the carbon dioxide absorber I, wherethe carbon dioxide content of the flue gas 4 is absorbed, with thewashed flue gas passing out 35 the absorber at the top through outletpipe 3 and discharged from the outlet 3 to the atmosphere, or to a plantfor the use of the resultant practically pure nitrogen gas. This is theextent of the gas cycle and it is essential that the temperature of thegas or circulation medium in the absorber I never exceeds 108 F. andpreferably said temperature should not exceed 90 F.

The carbon dioxide absorber I is a tower of sufficient height to permitthe completion of the reaction between the absorption medium and theacid gas with the diameter proportioned to keep a stated velocity of gaspassing through the apparatus. The base 4 of the absorber I is used as areservoir to compensate for fluctuations in the amount of circulation.

Starting at a point above the gas inlet 2 of the absorber I the absorberI is filled within a few feet of the top with wooden grids 5 or hurdlesmade of chemical stoneware or any other inert chamber and above thisspace 6 is a series of (117),v

' velocity of the gas is reduced thus minimizingmaterial which canbear-ranged to maintain a uniform distribution of the gas and liquor.Above the grids 5 is a gas space 6 utilized as a spray grids I identicalin construction to grids 5,- with a gas space 8 at the top of theabsorber I where the the probability of any water or absorption.solution being mechanically carried from the absorber l with.the gas.

In the base 4 oi! the absorber i the sulphur and I dirt free flue gas isintroduced through the inlet separating'irom the salt, uniting in thepresence ot water'with one molecule of carbon dioxide and formingammonium, bicarbonate, as expressed in the following equations:

-Na (NI-I4) HPO4+ (NH4) HCOa:

The mono sodium diammonium phosphate solution can be of any strengthfrom a very dilute solution up to a saturated solution at the temperature oi circulation. It is however preferable to use a solutioncontaining thirty-three and one-thirdparts of sodium diammoniumphosphate calculated as the anhydrous salt, (without water ofcrystallization) to one hundred parts .of water by weight. .This isapproximately a twenty-five percent solution based on the anhydrous salt(without water of crystallization). -I also prefer to have the amount ofmono sodium diammonium phosphate solution circulated slightly in excessof the theoretical requirements,

thus permitting a small proportion oi! ammonium carbonate to be formedas expressed in the following equation:

.ing coils l5, cooled by water flowing from an outside source 20 throughvalve 2| and sprays 22 over the outside surfaces of the coils i5. Andthe liquor then passes through pipe l6, valve i1 and pipe l3 to thebottom of the top ring 23 of,

extremely hot summer days and can be bypassed by opening valve l3 andclosing valves i3 and I7 when not essential for the maintenance or thedesired temperature.

The regenerator 24 as illustrated consists of eleven rings constructedof acid and alkali resisting metal, preferably of copper free castaluminum. y

.The three toprings 23 are dephlegmator rings, each ring 23 having .aphophone 25 through the floor 26, extending slightly above the liquorlevel v of said ring 23. Overthe phophone 25 isan arched passette 21 soarranged as to give ample passage for gas and vapors from said phophone25 with the passette 21 curving downward, so that the edge of thepassette 21 is substantially immersed below the surface of the liquor.This passette 21 separatesthe gas space of 'the ring 23 into two zones28 and 29. The space 28 under the passette being connected through thephophone 25 with the top gas space 23 of the ring 23 below. Thus allgasor vaporgeneratedin the ring 23 .has to bubble through the liquor inthe ring 23 above, thus heating the incoming liquor and cooling the gasand vapors.

Each of the three rings 23 is connected with the next adjacent ring 23by aninternal overflow 30,

the top-of which is slightly below the top of phophone 25 and extends towithin a short distance connects the gas space 28 of ring 23 with a topgas space 33 of ring 3i and the overflow 30 of said ring 23 extends towithin a'short distance of floor 38 of said ring 3|.

Below the dephlegmator ririgs 23 are six rings 3! filled with heatingtubes 32 so designed that the heating tubes 32 are at all times immersedin the circulatingliquor, with a small gas space 33 above a passette 34and which space is connect: ed by a phophone 35 to a gas space 36 on theunderside of the passette 34 in. the ring 3| above. The edge ofthearched passette 34 is slightly immersed in the liquor, thus separatingthe gas space into two separate and distinct zones 33 and 36 causing theescaping gas and vapor to -be washed in the downcoming liquor of thering 31 e1 of the top edge of phophone 35 and slightly above the top rowof tubes 32 and extends towithin a short distance of the floor 330i thenext low.-

'er' ring 3|. In each ring 3| are a number of rows of tubes 32, and thetubes 32 in each horizontal layer are connected .with a header and theheader so arranged that a number of adjacent tubes are in parallel andconnected in series with a similar number oi! tubes 32 in the samehorizontal layer.

The last series of parallel tubes 32 in any horizontal layer areconnected by a header to a sim-v ilar series of tubes 32 in the nexthorizontal layer tinued throughout the six rings 3i of the regenerator24 so that all the tubes 432 -oi' the sixrings 3| are in series ofparallel tubes.

The last two rings 39 of the regenerator 24 are tuberings similar indesignto the rings-3| above. with phophone 35 and overflow 31functioning as in rings 3! and connecting the lowest ring 3i to the ring39. The-heating tubes of rings 33 1 temperature. The water condensed insaid tubes bonate and sodium ammonium acid phosphate solution enters thetop dephlegmator ring 23 at a temperature of approximately 90 F. flowsdown in series through the three rings 23, the solution in passingthrough the rings 23 is slightly heated by the upcoming gas and vaporand then flows through in series the first six heating rings 3|, wherethe solution is further heated by the circulation in the tubes 32 of thehot efiluent from the base of the regenerator 24 and the solution thenpasses through the last two rings 39 in series where the circulationsolution is heated to a temperature sufiicient to dissociate theresidual ammonium carbonate'by means of the heat furnished by the livesteam in the heating tubes 40.

In the regenerator 24 several reactions take place. The ammoniumbicarbonate and carbonate are dissociated into ammonia and carbondioxide in the hot zones near the bottom of the apparatus and part ofthe free ammonia unites with the sodium ammonium acid phosphate to formmono sodium diammonium phosphate, as

expressed in the following equation:

2(NH )HCO 2NH 2CO 21-1 0 The balance of the dissociated ammonia and allof the carbon dioxide passing up from a hot zone of the regenerator 24to a cooler zone of the regenerator 24, tends to reunite formingammonium carbonate with part of the CO2 passing up and escaping from'theregenerator 24 through the outlet pipe 43 at the top of 24 which outletpipe leads to a container (not shown). This reaction can be expressed asfollows:

This ammonium carbonate formed dissolves in the downcoming liquor and isreturned to a hotter zone of the regenerator 24 where it is againdissociated into ammonia and carbon dioxide as follows:

(NI-I4)2CO3T *2NH3+CO2FH2O Part of the, ammonia unites with the'monosodium ammonium phosphate to form mono sodium diammonium phosphate,

N8(NH4)HPO4 Nlig= N8(NH4)PO4 and the balance passes up the regenerator24 to and returns to a hotter zone.

This dissociation and reforming of the carbonate continues until anequilibrium is estabform ammonium carbonate which is redissolvedlishedin the regenerator 24, whichvaries in accordance with the temperaturemaintained in the top ring 23 and the temperature maintained in the basering 39. I

With the maintenance of the carbon dioxide gas in the top ring 23 at atemperature of not to exceed 108 F. practically no ammonia vapor willpassed with the carbon dioxide gas and the gas will bepractically purecarbon dioxide saturated with water vapor at the temperature of exitthrough outlet pipe 43. However, with the gas temperature at the outlet43 above 108 F. some ammonia vapor will pass off with the carbondioxide. As-the temperature is raised at the outlet 43 more and more ofthe ammonia passes ofi with the carbon dioxide until with a gastemperature at the outlet 43 of approximately 184 F. and with the liquorin the base ring 39 of the regenerator 24 at a temperature in excess of210 F. substantially all of the ammonia and of the carbon dioxidecontent of the circulation liquor will pass ofl' through the outlet 43as a vapor from the regenerator 24.

Thus it will be seen that for the successful work- 7 ing of thisinvention it is essential that the-gas temperature at the outlet 43never exceeds 108 F. and the temperature of the circulating liquorleaving the regenerator 24 through outlet pipe 44 never exceeds atemperature sufliclent to completely dissociate the residual ammoniumcarbonate.

With the maintenance of a gas temperature atthe outlet 43 of 90 F. and aliquor temperature at outlet pipe 44 of 210 F. an equilibrium isestablished in the regenerator 24 with the liquor leaving theregenerator ,24 containing approximately twenty-five percent of theincoming ammonia which was combined with carbon dioxide, leaving asammonium carbonate. Thus in the first cycle of the circulation liquorapproximately eighty-seven and one-half percent of the carbon dioxideabsorbed in the absorber I will be eliminated in the regenerator 24 andin each subsequent cycle one hundred percent of the carbon dioxideabsorbed from the gas in the carbon dioxide absorber I will beeliminated in the regenerator 24. v

'Thus with the original circulation liquor containing the preferablestrength of approx mately 31.5 parts by weight of mono sodium diammoniumphosphate (calculated without water of crystallization) to one hundredparts of water by weight. the circulation liquor leaving the regenerator24 will contain by calculation approximately 25.12 parts by weight ofmono sodium diammonium phosphate, 7.45 parts of sodium ammonium acidphosphate and 2.50 parts of ammo? nium carbonate, and is the preferablesolution for the absorption of the carbon dioxide in the carbon dioxideabsorber I although it is to be understood that solutions of otherstrength either weaker or stronger, may be used, and this is thesolution that is referred to as a twenty-five percent solut on calcuated as mono sodium diammonium phosphate" in describing the absorber Icycle The preferable circulation liquor which is designated as atwenty-five percent mono sodium diammonium phosphate liquor isapproximately of the follow'ng composition:

Percent by Weight Mono sodium diammonium phosphate 18.55 Sodium ammoniumacid phosphate 5.52 Ammonium carbonate 1. 85 Water 74.08

But it is understood that a liquor of this compositi n is not essentialto the successful operation of this process but can be varied over widelimits without materially afiectlng the results 1 functions to assuresufllcient circulation for the removal of the carbon dioxide fromthe-flue gas,

and the limit switch functions to assure that the entire circulation isin equilibrium before any further impulses from the carbon dioxideindicator are answered by the pump 45.

The pump 45 forces the circulation liquor through pipe 46 to the heatingtubes 32 of the lowest ring 3i which is the third ring from the bottomof the regenerator 24, and the liquor flows through the six liquorheating rings 3i being cooled in its passage through the tubes 32 andtransferring its heat to the liquor in the body of the regenerator 24.The liquor leaves the.tubes 32 of the top heating ring 3i atapproximately 110 F. and then passes through pipe 41 to a series of twoinch cooling coils 48 cooled' by a stream of water flowing over theoutside surfaces of the coils 40 from an outside source 49, throughvalve 50 and sprays 5|. The circulation liquor now.cooled toapproximately 90 F. passes from cooling coils 48 through pipe 52 to acollector tank 53 broken away to show the interior construction and fromsaid tank 53 through pipe 54 is pumped by pump 55 through pipe 56 to thesprays 9 of the carbon diox de absorber i. This pump is synchronizedwith the pump at the base of regenerator 24. Any deficiency in thequantity of the circulation liquor is made up by introducing from anoutside source fresh solution of sodium diammonium phosphate intocollector tank 53 through valve 58 and pipe 55. Any excess of saidsolution of sodium diammonium phosphate over circulation requirements isremoved from collector tank 53 through pipe 60 and valve Bl. Thiscompletes the cycle of the absorption circulation medium.

The lower two heating rings 39 of the regen erator 24 are connectedthrough pipe 4| and valve 42 to an outside steam supply. The valve 42 isautomatically controlled by a temperature controller (not shown), thethermometer bulb of which is located in the gas space 29 of the topdephlegmator ring 23. This insures the maintenance of the proper anduniform temperature at all times.

The carbon dioxide gas liberated in the regenerator 24 passes up throughthe various rings 39, 3| and 23 and is washed by the downcoming sodiumammonium acid phosphate, which frees it of any entrainedammonia vaporand is cooled by the entering liquor and finally passes throughoutletpipe 43 to the point of consumption or to a storage holder (notshown).

The. carbon dioxide produced in accordance with the above description ispractically chemically pure, contains no ammonia vapor, contains noentrained water, but only the water of saturation at the temperature ofthe said gas leaving regenerator 24 through outlet pipe 43. Thereforethe carbon dioxide gas requires no extensive dehydration or coolingsystems toprepare it for subsequent processes. j

with the use of this process as described acid gases can be removed andrecovered with the use of "a comparatively small amount of externalheat. The amount of heat required is between 250 and 1000 Britishthermal units per pound of the acid gas recovered. This heat can befurnished'by means of live steam, exhaust steam or hot-industrial gases.The operation can be made entirely mechanical requiring very littlesupervision. .For a stated production of carbon dioxide dioxide recorder(not shownl on the gas outlet pipe 3' of the carbon dioxide'absorberz I.The

temperature of the regenerator 24 is maintained by a temperaturecontroller (not shown) It is to be definitely understood that whereas-,carbon dioxide is treated as the gas to be recovered. this processapplies to both organic and inorganic acid gases such as hydrogensulphide HzS, hydrocyanic acid gas HCN and mercaptans CnHznSi. It isalso to be definitely understood.

that any acid salt solution containing more than .two hydrogen ions canbe used, in the place of the mono sodium diammonium phosphate used as anexample. Also it is to be understood that the process is not restrictedto the use of sodium as of a mixed salt of the class consisting oftri-basic diammonium phosphates of the alkali metals and alkaline earthmetals and the subsequent recovery of the acid gas by regeneration ofthe mixed salt.

2. A process of purifying natural gas by removing acid gas constituentsby scrubbing the natural gas mixture with a solution of a mixed salt ofthe class consisting of tri-basic diammonium phosphates of the alkalimetals and alkaline earth metals and the subsequent recovery of the acidgas by regeneration of the mixed salt.

3. A process of purifying manufactured gas by removing acid gasconstituents by scrubbing the manufactured gas mixture with a solutionof a mixed salt of the class consisting of tri-basic diammoniumphosphates of the alkali metals and alkaline earth metals and thesubsequent recovcry of the acid gas by regeneration of the mixed salt.

. 4. A process for the removal of acid gases from industrial waste gasescontaining as a constituent acid gas which consists in scrubbing saidwaste gas with a solution of a mixed salt of the class consisting 'oftri-basic diammonium phosphates of the alkali metals and alkaline earthmetals which decomposes in the presence of an acid gas forming an acid.salt of the scrubbing medium and an acid salt of the acid gas and theremoval of the resultant solution from the scrubbing chamber to aheating chamber wherein the said acid salt of the acid gas is decomposedby heat with the acid gas passing from the top of the heating chamber toa holder and the base of the acid salt of the acid gas uniting with theacid salt of the scrubbing medium forming said mixed stituent bycontacting the same with sodium di-- ammonium phosphate andthesubsequent recovery of the acid gas by regeneration of the sodiumdiammonium phosphate. 6. A process of purifying, natural gas by removingacid gas constituents by scrubbing the natural gas mixture with asolution of sodium diammonium phosphate, and the subsequent recovery ofthe acid gas by regeneration of the sodium diammonium phosphate.

'7. A process of purifying manufactured gas by removing acid gasconstituents by scrubbing the manufactured gas mixture with a solutionof soacid gas from dium diammonium phosphate, and the subsequentrecovery 01 the acid gas by regeneration of the sodium diammoniumphosphate.

8. A process for the removal 0! acid gases from industrial waste gasescontaining as a constituent acid gas which consists in scrubbing saidwaste gas with a solution'oi sodium diammonium phosphate whichdecomposes in the presence 0! an acid gas forming an acid salt of thescrubbing medium and an acid salt or the volatile base andthe acid gasand the removal of the resultant solution from the scrubbing chamber toa heating chamber wherein the said acid salt of the acid 9.Aprocessoi'removingacidgasiromagueous mixture containing acid gas as aconstituent comprising scrubbing said gaseous mixture with a solution ota mixeds'alt of the class consisting of tri-basic diammonium phosphatesoi the alkali metals and alkaline earth metals, acid containing morethan two hydrogenions and the removal oi the resultant solution to aregeneration chamher where the temperature of the regenerator acid gasoutlet does not exceed 108 F.

10. A process of. removing acid gas from a gaseous mixture containingacid gas as a constituent comprising scrubbing said gaseous mixture witha solution of sodium diammonium phosphate and the removal of theresultant solution to a regeneration chamber where thetemp'erature oithe regenerator acid gas in the top zone does not exceed 108 F.

WALTER R. KNAPP.

